Photoreceptors and phototransductioncvrl.ucl.ac.uk/neur0017/Lecture...

NEUR0017 Visual Neuroscience

Photoreceptors and phototransduction

Andrew Stockman

Background

Light 400 - 700 nm is important for vision

The retina is carpeted with light-sensitive rods and cones

An inverted image is formed on the retina

The eye

Retinal cross-section

Retina 200 ×

Light

Webvision

Rods and cones

Human photoreceptors

Rods Achromatic

night vision 1 type

Short-wavelength-sensitive (S) or “blue” cone

Middle-wavelength-sensitive (M) or “green” cone

Long-wavelength-sensitive (L) or “red” cone

Cones Daytime, achromatic

and chromatic vision 3 types

Rod

0.3 mm of eccentricity is about 1 deg of visual angle

The central foveola (c. 1.25 deg diam.) is rod free

Human photoreceptor mosaics

The light-sensitive photopigment lies inside the

rod and cone outer segments.

Light

Light

Light and the eye

Arrangement of visual pigment molecules

Human foveal cones each contain about 1010 visual pigment molecules

The molecule consists of protein, opsin, forming 7 transmembrane α-helices, surrounding the chromophore, retinal, the aldehyde of Vitamin A

(chromo- colour, + -phore, producer)Light-catching portion of any molecule

11-cis retinal The molecule is twisted at the 11th carbon.

Chromophore


A photon is absorbed11-cis retinal

Chromophore


A photon is absorbed11-cis retinal

the energy of which initiates a conformational change to…

Chromophore


A photon is absorbed

OFF

ON


11-cis retinal

all-trans retinal OFF

ON

Think of the molecule as a photo-sensitive switch!

Chromophore

PhototransductionEnergy of absorbed photon is converted (transduced) to

an electrical neural signal, the receptor potential.

Inspired by:Pugh, Nikonov, & Lamb (1999). Current Opinion on Neurobiology, 9, 410-418.Burns & Arshavsky (2005). Neuron, 48, 387-401.

Phototransduction• Activation• Range extension• Deactivation

Rhodopsin (R)G-protein

coupled receptor

PDE6Phosphodiesterase,

effector molecule, withα, β and two γ subunits

Transducin (G)G-protein, trimer with α, β and γ subunits

Chromophore11-cis-retinal

Main molecular players in the cascade

In the Dark…

In the Dark

Intracellular cGMP concentration highCNG channels open

CNG = Cyclic Nucleotide Gated channel

Ionpump

CNGchannel

Activation steps

A photon is absorbed resulting in activated R*

Activation steps

Activated transducin, G*α, binds to and activates PDE6 by displacing the γ inhibitory subunit to

produce G*α-PDE6*.R* catalyses the exchange of GDP for GTP on the G-protein,

producing the activated subunit G*α, which dissociates

Activation steps

G*α-PDE6* produces a local drop in cytoplasmic cGMP

The drop in cGMP leads to closure of the CNG channels, which blocks the entry of Na+ and Ca2+ ions into the outer

segment, causing the outer segment to hyperpolarize.

Activation steps

How many photons are needed for us to detect light (when fully dark-adapted)?

When fully dark-adapted, we can detect as few as 7-10 photons.

How is this possible?

Amplification

A single R* catalyses the activation of c. 500 transducinmolecules. Each G*α can stimulate one PDE6*, which in turn can break down 103 molecules of cGMP per second. Thus, a single R* can cause the hydrolysis of >105 molecules of cGMP per second!

The absorption of a single photon is sufficient to change the membrane conductance. How?

Amplification is beneficial at low light levels, but what negative effects might amplification have at high light levels?

An important function of the photoreceptor and the transduction cascade is:

Range extension andlight adaptation

Why is light adaptation or sensitivity regulation important?

Photopic retinal illuminance(log phot td)

Scotopic retinal illuminance(log scot td)

-4.3

-3.9 -2.0 -0.1 1.5 3.1 4.9 6.9 8.9

-2.4 -0.5 1.1 2.7 4.5 6.5 8.5

Typical ambient light levelsIndoorlighting

Starlight

Moonlight Sunlight

Because the visual system must maintain itself within a useful operating range over the roughly 1012 change in illumination: from absolute rod threshold to levels at which photoreceptor damage can occur.

Absolute rodthreshold

Conethreshold

Rod saturationbegins

Damagepossible

Visual function

It must do so despite the fact that that a typical postreceptoral neuron can operate over a range of only c. 103.



-4.3

-3.9 -2.0 -0.1 1.5 3.1 4.9 6.9 8.9

-2.4 -0.5 1.1 2.7 4.5 6.5 8.5


Starlight

Moonlight Sunlight


Conethreshold


Damagepossible

Visual function

SensitiveROD SYSTEMLower range

Less sensitiveCONE SYSTEMUpper range

Rods that are optimized for low light levels


Starlight

Moonlight Sunlight


Conethreshold


Damaginglevels

Visual function

Cones that are optimized for higher light levels

Rods and cones



-4.3

-3.9 -2.0 -0.1 1.5 3.1 4.9 6.9 8.9

-2.4 -0.5 1.1 2.7 4.5 6.5 8.5


Starlight

Moonlight Sunlight

PHOTOPIC


Conethreshold


Damagepossible

SCOTOPICVisual function

MESOPIC

Scotopic levels(below cone threshold)

where rod vision functions alone.A range of c. 103

Photopic levels(above rod saturation)

where cone vision functions alone.A range of > 106

Mesopic levelswhere rod and cone

vision function together.

A range of c. 103

Rods and cones

Adaptation and sensitivity…

System must ADAPT to changes in light level

Ideally, the system should be very sensitive at low light levels, so that it can detect a few photons, but then much, much less sensitive at high light levels.

How can this achieved within the transduction cascade?

At low light levels the sensitivity is very high: A single R* can cause the hydrolysis of >105 molecules of cGMP per second!

But as the light level increases, the system will saturate (as you run out of “stuff”).

Adaptation and sensitivity…

Reduction in [Ca2+] causes Calmodulin (CaM) to dissociate from the CNG channels raising the affinity of the channels for cGMP

Ca2+ feedback

Range extension (1)

Reduction in [Ca2+] causes Calmodulin (CaM) to dissociate from the CNG channels raising the affinity of the channels for cGMP

Reduction in [Ca2+] causes dissociation of Ca2+ from

GCAP, allowing it to bind to GC increasing the rate of

resynthesis of cGMP

Ca2+ feedback

Range extension (2)

Increase in concentration of G*α-PDE6* in light speeds up rate of reaction 2 and speeds up the visual response

Adaptation: Speeding up the visual response

How does speeding up the visual response help light adaptation?

It reduces the integration time of the system...

Time

Inpu

t

Lightadaptation

Long integration time

Short integration time

Time

Outpu

t

Time

Out

put

Time

Out

put

Lightadaptation

What are the benefits of this type of adaptation?

Time

Inpu

t

Lightadaptation



Time

Outpu

t

Time

Out

put

Time

Out

put

Lightadaptation

Deactivation

Speeding up deactivation also decreases temporal integration.

Rec-2Ca2+ forms a complex with RK, blocking its activity.

When [Ca2+] drops, Ca2+

dissociates and Rec goes into solution.

Deactivation steps

Ca2+ feedback

Free RK multiply phosphorylates R*

Ca2+ feedback

Deactivation steps

Arrestin (Arr) quenches the phosphorylated R*

Ca2+ feedback

Deactivation steps

RGS9 deactivates the activated G*α-PDE6* complex.

(2)

Deactivation steps

Deactivation steps

Second run through…

Phototransduction cascade activation stages

Credit: Pugh, Nikonov & Lamb

Activation steps of the phototransduction cascade

A photon is absorbed resulting in activated R*



R* catalyses the exchange of GDP for GTP on the G-protein, producing the activated

transducin, subunit G*α (Gα-GTP).



Activated transducin, G*α, in turn, binds to and activates phospho-diesterase (PDE6) by displacing γ

inhibitory subunits to produce PDE6*.



PDE6* (G*α-E*) activity produces a local drop in cytoplasmic cG (cGMP)



A drop in cGMP leads to closure of cGMP gated channels, blocking the entry of Na+ and Ca2+ into

the outer segment. The ion exchanger continues to function lowering [Ca2+] in the outersegment.

Phototransduction cascade inactivation steps


Inactivation steps of the phototransduction cascade

Removal of Ca2+ activates guanylate cyclase activating protein, GCAP. Activated GCAP binds to guanylate cyclase, stimulating production of cG.

Ca2+ feedback


In the dark, when [Ca2+] is high, most of recoverin (Rec) is in the calcium bound form at the membrane; Rec-2Ca2+ forms a complex bond with rhodopsin kinase (RK) blocking its activity.

Ca2+ feedback


When [Ca2+] drops, Ca2+ dissociates from Rec, which moves into solution. Free RK rapidly increases, increasing its interaction with R*, and leading to its rapid phosphorylation.

Ca2+ feedback


Arrestin (Arr) then binds quenching the activity of R*.

Ca2+ feedback


G*α-E* is inactivated when the terminal phosphate of its bound GTP is hydrolyzed, which occurs when

the RGS9-Gβ5 protein binds to the complex.

Summary of molecularadaptation mechanisms

We’ll come back to these in the Sensitivity Regulation lecture

Mechanisms that shorten the visual integration time

[Ca2+] dependent activity of Rec

[G*α-PDE6*] dependent Increased rate of hydrolysis of cGMP to GMP

Changing the integration time of the system...

Time

Inpu

tLightadaptation



Time

Outpu

t

Time

Out

put

Time

Out

put

Lightadaptation

Time

Inpu

t

Time

Inpu

t

INPUT OUTPUT

Shortening the integration time of the system increases sensitivity to higher flicker rates...

Lightadaptation


Time

Inpu

t

Time

Inpu

t

Out

put

Time

Time

Out

put


INPUT OUTPUT

Human temporal response

An excellent way of characterizing the effects of light adaptation psychophysically is to measure changes in the temporal response.

Focus on changes in temporal sensitivity.

0 10 20 30 40

-1

0

1

2

3

4

5 MM

Frequency (Hz)

0 10 20 30 40

Log

thre

shol

d am

plitu

de (l

og td

)

-1

0

1

2

3

4

5 ML

LEVELS (log td)

5.695.284.754.163.39

2.79 2.20 1.60 1.05 0.42

Changes in temporal

sensitivity

Adap

tatio

n le

vel i

ncre

asin

g do

wnw

ards

Substantial relative improvements in

sensitivity at moderate and high temporal

frequencies

Reduction in the number of open CNG-gated channels

Photopigment bleaching (less photopigment available at high

light levels)

Mechanisms that simply decrease sensitivity

Changing the gain (attenuation) of the system...

Time

Inpu

tLightadaptation

High gain(low attenuation)

Low gain(high attenuation)

Time

Outpu

t

Lightadaptation

Time

Inpu

t

Time

Inpu

t

INPUT OUTPUT

Time

Outpu

t

Time

Outpu

t

Phototransduction – cones versus rods

Visual pigment, transducin, arrestin PDE6, cGMP channel, and recoverin.

Quantitative differences. In cones:(i) R* forms 4 times faster than for rods - faster onset of light response.

(ii) R* decays 10-50 times faster (lower amplification factor).

(iii) GTPase activating protein (RGS-Gβ5) expressed at much higher levels -shorter G*α (activated transducin) lifetime - faster recovery.

(iv) Clearance of Ca2+ from cone outer segments is several times faster than for rods.

(v) cGMP channels in cones are twice as permeable to Ca2+ than in rods.

Cones have different isoforms of:

Cones versus rods

Cones are 25 - 100 times less sensitive to single photons.

They catch fewer photons (less visual pigment).

They respond with faster kinetics (isoforms of transduction cascade).

They have a much greater ability to adapt to background light.

They do not saturate at normal environmental light levels.

Cones versus rods

TRANSDUCTION AND UNIVARIANCE


A photon is absorbed

OFF

ON


11-cis retinal


ON

Think of the molecule as a photo-sensitive switch!

Chromophore

OFF

ON

11-cis retinal


ON

Crucially, the event is binary or “all or nothing”.

If a photon is absorbed it has the same effect as any

other absorbed photon, whatever its wavelength.

Chromophore

OFF

ON

11-cis retinal


ON

SAME EFFECT




Chromophore

OFF

ON

11-cis retinal


ON

SAME EFFECT




Chromophore

OFF

ON

11-cis retinal


ON

SAME EFFECT




Chromophore

OFF

ON

11-cis retinal


ON




SAME EFFECT

Chromophore

OFF

ON

11-cis retinal


ON

Can this process encode wavelength (colour)?

Chromophore

OFF

ON

11-cis retinal


ON

No, it cannot encode wavelength (colour)!

It is “UNIVARIANT”

Chromophore

“UNIVARIANT”

What does univariance mean in practice?

Use Middle-wavelength-sensitive (M) cones as an example…

Vision at the photoreceptor stage is relatively simple because the output of each photoreceptor is:

M-cone

So, if you monitor the cone output, you can’t tell which wavelength (energy) of photon has been absorbed.

UNIVARIANCECrucially, the effect of any absorbed photon is independent of its wavelength.

M-cone

UNIVARIANCECrucially, the effect of any absorbed photon is independent of its wavelength.

All the photoreceptor effectively does is to count photons.

What does vary with wavelength is the probability that a photon will be absorbed.

This is reflected in what is called a “spectral sensitivity function”.

UNIVARIANCE

Wavelength (nm)400 450 500 550 600 650 700

Log 10

qua

ntal

sen

sitiv

ity

-3

-2

-1

0

Imagine the sensitivity to these photons…

> >> > > >In order of M-cone sensitivity:

Wavelength (nm)400 450 500 550 600 650 700

Log 10

qua

ntal

sen

sitiv

ity

-3

-2

-1

0

If you have four lights of the same intensity (indicated here by their heights), how will they look?

The green will look brightest, then yellow, then blue and lastly the red will be the dimmest

Wavelength (nm)400 450 500 550 600 650 700

Log 10

qua

ntal

sen

sitiv

ity

-3

-2

-1

0

If we adjust the intensities to compensate for the sensitivity differences, how will they look?

The four lights will look completely identical.

M-cone

Changes in light intensity are confounded with changes in colour (wavelength)

Vision at the photoreceptor stage is relatively simple because the output

of each photoreceptor is:

UNIVARIANT

If we had only one photoreceptor type in our eyes, what colours would we see?

We would be colour-blind…

Examples: night vision, blue cone monochromats

Univariance

If a cone is n times less sensitive to light A than to light B, then if A is set to be n times brighter than B, the two lights will appear

identical whatever their wavelengths.

What does vary with wavelength is the probabilitythat a photon will be absorbed, and this relationship is different for the four different photoreceptors.

This is reflected in the photoreceptor spectral sensitivity functions…

Wavelength (nm)400 450 500 550 600 650 700

Log 10

qua

ntal

sen

sitiv

ity

-5

-4

-3

-2

-1

0

Four human photoreceptors have different spectral sensitivities

λmax (nm, corneal, quantal)441 500 541 566

Note logarithmic scale

L

MS

Rods

So, if each photoreceptor is colour-blind, how do we see colour?

Or to put it another way: How is colour encoded at the input

to the visual system?

Colour is encoded by the relative cone outputs

Wavelength (nm)

400 450 500 550 600 650 700

Log 10

qua

ntal

sen

sitiv

ity

-3

-2

-1

0

S ML

Blue light

Blue light

Green light

Wavelength (nm)

400 450 500 550 600 650 700

Log 10

qua

ntal

sen

sitiv

ity

-3

-2

-1

0

S ML


Wavelength (nm)

400 450 500 550 600 650 700

Log 10

qua

ntal

sen

sitiv

ity

-3

-2

-1

0

S ML


Blue light

Red light

Green light

Blue light Red light

White lightYellow light

Green light Purple light


Because there are three univariant cones in the eye, human colour vision is a three-variable “trichromatic” system that depends on the relative outputs of the three cones.

A change in colour from green to red causes a relative increase in the L-cone output but causes a decrease in the M-cone output.

Thus, colour can be encoded by comparingthe outputs of different cone types…

TRICHROMACY

A change in colour from red to green causes a relative increase in the M-cone output but causes a decrease in the L-cone output.

M

M

L

L

Photopigments and spectral tuning

The spectral sensitivities of the photopigments depend on the energy required to initiate the rotation of the chromophore from its 11-cis form to its all-trans form.

Photopigment molecule (cone)From Sharpe, Stockman, Jägle & Nathans, 1999

So how is the initiation energy modified?

But the same chromophore is used in all four human photo-pigments.

Photopigment molecule (cone)From Sharpe, Stockman, Jägle & Nathans, 1999

The initiation energy is altered by changing the amino acids in the surrounding opsin molecule.

But the same chromophore is used in all four human photo-pigments.

From Sharpe, Stockman, Jägle & Nathans, 1999

There are only 15 amino acid differences between L and M: 96% identical

Opsin differences

POLA

RN

ON

-PO

LAR

LWS all with OH group

MWS

180 277 285

Three main amino acid differences are responsible for the spectral sensitivity

difference between M and L.

MWS LWS

Tuning Site

180 alanine serine OH- ~ 5 nm

277 phenylalanine tyrosine OH-

285 alanine threonine OH-

~ 25 nm

Amino acid differences

From Sharpe, Stockman, Jägle & Nathans, 1999

Why are the M- and L-cone opsins so similar?

Phylogenetic tree of visual pigments

About 460 – 520 nm Rh1

About 460 – 520 nm Rh2

About 355 – 450 nm SWS1

About 490 – 570 nm LWS

About 420 – 480 nm SWS2

490 - 500 nm

Mammals

lost

lost

365 - 450 nm

490 - 565 nm

Basis for dichromatic colour vision

Credit: Bowmaker

490 - 500 nm

Humans

lost

lost

365 - 450 nm

Gene duplication

Basis for trichromatic colour vision

Credit: Bowmaker

Gene duplication on the X-chromosome

L-cone photopigment

opsin gene

M-cone photopigment

opsin gene

Mammal

Human/ Old world primate

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